1. Field of the Invention
The present invention relates to a differential amplifier, a sample-and-hold circuit, and an amplifier circuit.
2. Description of the Related Art
In a conventional differential amplifier, only one of an input common-mode voltage and an output common-mode voltage of the differential amplifier is controlled to set a direct-current operating point of the differential amplifier. To perform a low-voltage operation of the differential amplifier without narrowing an output voltage range of the differential amplifier, there are methods of operating the differential amplifier as a grounded source amplifier by omitting a current source commonly connected to source sides of transistors constituting a differential pair.
For example, U.S. Pat. No. 6,140,877 discloses a method of controlling an output common-mode voltage by feeding back the output common-mode voltage to a back gate terminal of a signal input MOSFET.
U.S. Pat. No. 6,677,822 discloses a method of controlling an output common-mode voltage by adjusting a bias current of a grounded source amplifier based on the output common-mode voltage.
Japanese Patent Application Laid-open No. 2006-174033 discloses a method of controlling an output common-mode voltage by adjusting a source potential of a grounded source amplifier based on the output common-mode voltage.
Japanese Patent Application Laid-open No. H10-322143 discloses a method of correcting a bias setting of an operating point and a direct-current (DC) offset by performing common-mode negative-feedback control to a fully differential amplifier that is provided by combining a transconductance circuit, a constant gain amplifier, and a capacitor by using these as constituent elements.
However, according to the method disclosed in U.S. Pat. No. 6,140,877, an adjustable range of an output common-mode voltage is only a variable range of a threshold voltage of the MOSFET of a grounded source amplifier due to a back-gate effect. Therefore, this method has a problem that a sufficient adjustable range is not necessarily obtained.
According to the method disclosed in U.S. Pat. No. 6,677,822, transconductance of a transistor changes when a bias current is adjusted. Therefore, this method has a problem that a frequency characteristic of a grounded source amplifier changes, and a desired frequency characteristic cannot be obtained necessarily.
According to the method disclosed in Japanese Patent Application Laid-open No. 2006-174033, a transistor is connected to a source of a grounded source amplifier to adjust a source potential of the grounded source amplifier. Therefore, this method has a problem that an output voltage range becomes narrow corresponding to a voltage drop of the transistor.
According to the method disclosed in Japanese Patent Application Laid-open No. H10-322143, an output common-mode voltage is fed back to a signal input terminal of an amplifier via a transconductance circuit. Therefore, this method has a problem that thermal noise and flicker noise generated in the transconductance circuit are differentially amplified as they are, and a differential noise characteristic is degraded.